The present invention relates to a receiver in a wireless communication system, and more particularly, to a polarization diversity receiver which aligns the phase of received cross-polarized signals and combines the received signals to generate an enhanced received signal.
In wireless communications systems, transmitted signals become substantially degraded before being received by a receiver. Received signals are a combination of the transmitted signal along a direct path between the transmitter and receiver as well as reflections of the transmitted signal by intervening objects such as terrain and buildings and atmospheric conditions such as inverse gradients of relative humidity and temperature.
The orientation of the electric field vector of the transmitted signal is referred to as the signal""s polarization. The polarization of the transmitted signal changes as it is reflected to the receiver. Due to reflections, the received signal can become orthogonally polarized to the transmitted signal. The strength of a signal which is received by an antenna is highest when the received signal has the same polarization (i.e. co-polarized) as the antenna and becomes substantially zero when the received signal is orthogonally polarized to the antenna. Consequently, depolarization of the transmitted signal can result in signal xe2x80x9cholesxe2x80x9d where the signal strength drops below a minimum threshold of reception by the receiver.
To compensate for depolarization of the transmitted signal, some receivers are coupled to a pair of antennas which have orthogonal polarizations, such as linear polarizations (e.g. horizontal and vertical), elliptical polarizations (e.g. horizontal and vertical major axes), or circular polarizations (e.g. right circular and left circular). The phase centers of the antennas can be co-located so that only one antenna structure is needed on the receiver. Improved signal reception is provided by the receiver selecting and then processing the strongest one of the two signals from the antennas.
Multiple reflections of a transmitted signal constructively and destructively superpose at the receiver according to the relative phase and amplitude of all of the reflected signals. The interaction of reflected signals produces both strong peaks (local maximums) and deep nulls (local minimums) of relative signal strength relative to different locations of the receiver. As with depolarization of the transmitted signal, multipath relections of the transmitted signal can result in signal xe2x80x9cholesxe2x80x9d where the signal strength drops below the minimum reception threshold of the receiver. Depending on the relative phase of the interacting signals, the peaks and nulls of signal strength are generally separated by a fraction of the wavelength of the carrier frequency of the transmitted signal.
To compensate for multipath signal fading, some receivers are coupled to a pair of antennas which are separated by a minimum of one half of a wavelength of the carrier frequency of the transmitted signal. By separating two receiving antennas by the appropriate distance, each antenna receives a signal whose fading pattern can be uncorrelated with the fading pattern of the signal received by the other antenna. As with polarization diversity reception, improved signal reception is provided by the receiver selecting and then processing the strongest one of the two signals from the antennas.
By selecting and processing only the strongest one of the two signals which are received by the antennas, the electromagnetic energy of the weaker signal is discarded. Consequently, the strength of the received signal continues to be limited by depolarization and multipath effects on the transmitted signal.
The present invention is directed to a receiver and an associated method which provides a strong received signal irrespective of any depolarization of the received signal. The receiver is used with a first antenna which receives first polarized signals and a second antenna which receives second polarized signals, where the first polarized signals have a substantially different polarization than the second polarized signals.
The receiver includes a phase shifter and a combiner circuit. The phase shifter adjusts the phase of the first polarized signal in response to the phase of the second polarized signal to produce a phase compensated first signal. Preferably, the phase shifter adjusts the phase of the first polarized signal to be substantially aligned with the phase of the second polarized signal. The combiner circuit sums the second polarized signal and the phase compensated first signal to generate a combined polarization received signal. In this manner, the depolarized signals and polarized signals are phase-aligned and then combined to generate an enhanced received signal irrespective of the depolarization characteristics of the received signal.
In a further embodiment of the present invention, spatial diversity reception is combined with polarization diversity reception by separating the first and second differently polarized antennas by at least about one half of a wavelength of the carrier frequency of the transmitted signal. The received signals are phase-aligned and then combined to generate an enhanced received signal irrespective of the multipath characteristics and the depolarization characteristics of the received signal.